Electronic Design

Avoid Transformer Core Saturation

In certain situations, a 50% dutycycle signal can pass through a transformer without problems, providing there's a negative half-cycle to completely demagnetize the core. However, if the core isn't wholly demagnetized, a buildup of energy can occur, and eventually the core saturates and the pulse no longer can pass through the transformer. In general, low frequencies aren't a problem because the core is only magnetized for a small part of the pulse period. But high frequencies, typically those used in microprocessor-timing and logic circuits, can cause saturation problems in the core due to magnetic energy build-up, particularly when transitions are between zero and a positive voltage.

This setup (Fig. 1) overcomes the problem of passing 50% duty-cycle signals through an isolating transformer. It sends narrow edge-triggered pulses through the transformer and regenerates the 50% signal on the other side of the isolation barrier. The circuit consists of a positive-and negative-edge trigger, an OR-gate logic driver, and a D-type flip-flop divider. The edge detectors produce narrow pulses. Their pulse width can be determined by selecting the appropriate R and C values (tp ≈ 2ΠRC). The narrow pulses are combined at the OR gate to yield a frequency-doubled signal with a narrow pulse width. Pulse widths of less than 50% allow the core to easily desaturate during the "off" cycle. The 50% duty cycle is regenerated using a D-type flip-flop frequency divider connected on the other side of the isolation barrier.

Using a 500-kHz signal and standard 74LS logic, timing results (Fig. 2) were produced for the resistance and capacitance values shown in the circuit of Figure 1. Higher frequencies are accommodated by reducing the capacitor's value and, if necessary, using faster logic gates. This circuit's most likely applications would be in data bus isolators where data exchange is at high frequency, and when passing a clock signal through an isolator.

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